1. FIELD OF THE INVENTION
The present invention relates to an improved method for the preparation of crystalline silicate molecular sieves. More particularly, the crystalline silicates are prepared from reaction formulations that use zeolitic aluminosilicates having sodalite substructures and has a tetrahedra atom density of less than about 15 TO.sub.2 per 1000 .ANG..sup.3 as the aluminum and silica source.
2. DESCRIPTION OF RELATED ART
Natural and synthetic zeolitic materials have demonstrated catalytic properties for use in various types of hydrocarbon processes. By using X-ray diffraction, certain zeolitic materials have been shown to have a crystalline structure wherein there are a large number of smaller cavities These smaller cavities may be interconnected by a number of still smaller channels or pores. Variations in pore dimensions and shapes have been found to effect the adsorptive and catalytic properties of the zeolites. Only molecules of certain dimensions and shapes are able to fit into the pores of a particular zeolite while molecules of a larger dimension or of a different shape are unable to enter the pore in the zeolite crystals. Since the pores in the zeolite accept molecules of certain dimensions while rejecting those of a larger dimension, these zeolites have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. These cavities and pores have been demonstrated to be of a uniform size within a specific zeolite.
Because of their unique molecular sieving characteristics, as well as their potentially acidic nature, zeolites are especially useful in hydrocarbon processing as adsorbents and as catalysts for cracking, reforming, and other hydrocarbon conversion reactions.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigged three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked through the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example by the inclusion of an alkali metal or an alkaline earth metal cation. This can be expressed by the relationship of aluminum to the cations, wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K, Cs or Li, is equal to unity. One cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques well know to those skilled in the art. By suitable selection of the cation, it has been possible to vary the properties of a given aluminosilicate.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolites have come to be designated by letter or other convenient symbols, for example: zeolite A (U.S. Pat. No. 2,882,243 issued on Apr. 14, 1959 to Milton), zeolite X (U.S. Pat. No. 2,882,244 issued on Apr. 14, 1959 to Milton), zeolite Y (U.S. Pat. No. 3,130,007 Apr. 21, 1964 to Breck), zeolite ZK-5 (U.S. Pat. No. 3,247,195 issued on Apr. 19, 1966 to Kerr), zeolite ZK-4 (U.S. Pat. No. 3,314,752 issued on Apr. 18, 1967 to Kerr), and SSZ-16 (described in U.S. Pat. No. 4,508,837 issued to Zones on Apr. 2, 1985.
Generally, crystalline silicates are synthesized by contacting an active source of one or more oxides selected from the group consisting of monovalent element oxides, divalent element oxides, trivalent element oxides, and tetravalent element oxides, an organic template, and water. Then, under conditions suitable for crystallization and with specific ratios between the precursors of the zeolites, the desired zeolite can be crystallized.
It has been found that crystalline components are generally not good reagents in zeolite synthesis. Occasionally, there are exceptional materials which turn out to be very useful reactants. This may come as a result of such features as very high surface area for the crystalline material, or there may be an unusual instability in the lattice framework. It is also thought that very small crystal size can also be a helpful feature.
It was previously shown in U.S. Pat. No. 4,689,207 that a crystalline silica, Magadiite was a useful reactant for high silica zeolite synthesis. U.S. Pat. No. 4,503,024 also discloses a method of preparing chabazite, merlinoite, edingtonite, ZSM-5, and ZSM-11 from the natural and synthetic zeolites mordenite, ferrierite, clinoptilolite, zeolite X and zeolite Y. The entire disclosures of each being incorporated herein by reference.
For a given zeolite structure it is not always possible to prepare the zeolite over a wide SiO.sub.2 /Al.sub.2 O.sub.3 compositional range. While the zeolite is largely composed of SiO.sub.2, it is the substitution of aluminum for silica which imparts acid characteristics to the zeolites. In attempting to modify the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio to a desired range, the prior art teaches several methods for replacing framework aluminum with other elements. These methods largely comprise techniques where alumina is removed from the crystalline zeolite structure thereby reducing the acidity of the zeolite. U.S. Pat. No. 5,098,687 discloses a process for removing framework aluminum from zeolites and substituting iron and/or titanium. Other methods include ion exchanging the aluminum using acid or base or combinations of both methods.
Examples of the above methods include: U.S. Pat. No. 3,620,960 (treatment of the zeolite with molybdenum fluoride); U.S. Pat. No. 3,630,965 (treatment of the zeolite with hydrofluoric acid); U.S. Pat. No. 3,644,220 (treatment of the zeolite with volatile halides selected from the group consisting of aluminum, zirconium, titanium, tin, molybdenum, tungsten, chromium, vanadium, antimony, bismuth, iron, platinum group metals and rare earths); U.S. Pat. No. 3,575,887 and U.S. Pat. No. 3,702,312 (treatment of the zeolite with fluorides and chlorides). U.S. Pat. No. 3,699,056 (treatment of the zeolite with halogenated hydrocarbons); U.S. Pat. No. 4,427,788 (ammoniacal aluminum fluoride solution for treatment of zeolite having silica/alumina ratio greater than 100).
Applicants have now unexpectedly discovered that it is possible to prepare a molecular sieve from a reaction mixture without necessarily resorting to subsequent dealumination process steps.
It is an object of the invention to provide a method for synthesizing a variety of molecular sieves using, as a reagent, another zeolite as the sole source of alumina and silica. The source zeolite has a faujasitic type structure with a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio greater than 200.
This and other objects are accomplished by the invention below.